Earth and Planetary Science Letters 233 (2005) 429–440 www.elsevier.com/locate/epsl Primitive neon isotopes in Terceira Island (Azores archipelago) Pedro Madureiraa,b,T, Manuel Moreiraa, Joa˜o Matac, Claude Jean Alle`grea aLaboratoire de Ge´ochimie et Cosmochimie, Insitut de Physique du Globe de Paris-Universite´ Paris VII-Denis Diderot, 4 place Jussieu, 75005, Paris, France bCentro de Geofı´sica de E´ vora/Departamento de Geocieˆncias da Universidade de E´ vora, Rua Roma˜o Ramalho, 59,7000-671, E´ vora, Portugal cCentro e Departamento de Geologia da Universidade de Lisboa, Faculdade de Cieˆncias, Campo Grande, Edifı´cio C6,48 Piso, 1749-016, Lisboa, Portugal Received 27 July 2004; received in revised form 27 January 2005; accepted 15 February 2005 Available online 13 April 2005 Editor: K. Farley Abstract We present the first neon data, as well as new helium data, on Terceira Island (Azores archipelago, Portugal). Clear 20Ne and 21Ne excesses compared to air are observed (20Ne/22NeN11.2) and moreover, the samples show a more primitive 21Ne/22Ne ratio than MORB, confirming that the Azores hotspot can be considered as sampling a bprimitiveQ, relatively undegassed, reservoir. Most 4He/3He isotopic ratios range between 80,000 and 63,500 (~ 9 to 11.5 R/Ra), being similar to those previously reported by [1] [M. Moreira, R. Doucelance, B. Dupre´, M. Kurz, C.J. Alle`gre, Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169 (1999) 189–205]. A more primitive 4He/3He ratio of ~ 50,000 (R/Ra ~ 15) was obtained in one sample, but we cannot completely exclude the possibility of a cosmogenic 3He contribution for this sample. Our study illustrates that the neon systematics can be more capable than helium to constrain the ultimate origin of hotspots in geodynamic settings dominated by plume–ridge interaction. D 2005 Elsevier B.V. All rights reserved. Keywords: neon; helium; Azores; plume–ridge interaction 1. Introduction emerged for the MORB and OIB sources (e.g. [2,3] and references therein). 4He production results from From the collection of helium isotopic data in the radioactive decay of U and Th and different time- last 20 yrs, significantly different 4He/3He signatures integrated isotopic ratios can be produced by hetero- geneous mantle degassing, through distinct depletion of He as compared with U and Th. MORB samples, T ´ Corresponding author. Centro de Geofisica de Evora/Departa- considered to come from the upper mantle, present a mento de Geocieˆncias da Universidade de E´ vora, Rua Roma˜o 4 3 Ramalho, 59,7000-671, E´ vora, Portugal. Tel.: +351 266745300; narrow range of He/ He ratios with a mean value fax: +351 266745397. around 85,000 to 90,000 (R/Ra=8 to 8.5; where Ra is 3 4 6 E-mail address: [email protected] (P. Madureira). the atmospheric He/ He ratio of 1.384Â10À ). A 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.02.030 430 P. Madureira et al. / Earth and Planetary Science Letters 233 (2005) 429–440 significantly larger variation is found for OIB, but to identify the incorporation of relatively undegassed 4He/3He ratios as low as 20,500 (R/Ra ~ 35) and lower mantle portions in plumes. The Azores was 15,000 (R/Ra ~ 50) measured in samples from Loihi firstly classified as one of the blow 3He hotspotsQ seamount and Iceland hotspot (Baffin island) respec- characterized by 4He/3He ratios higher than the mean tively, are interpreted as reflecting the contribution of MORB ratio [32]. However, Moreira et al. [1] showed a more primitive and less degassed source believed to that Azores Islands have 4He/3He ratios both higher be the lower mantle [4–8]. This interpretation is in and lower than MORB values. Higher values agreement with the geophysical evidence linking the (4He/3HeN140,000; R/Rab5.2) are represented by hotspot activity in these areas to deeply rooted mantle rocks from the Eastern part of S. Miguel, while more plumes [9–11]. The existence of these 4He/3He primitive ratios were recorded in Terceira samples contrasts led to the proposition of a two-layer mantle (around 64,000; R/Ra=11.3). Using Pb isotopic data, model where the upper mantle is degassed, while a these authors considered the S. Miguel end-member relatively undegassed reservoir is sampled by deep ( 206Pb/204Pb=20.00; 207Pb/ 204Pb= 15.75; mantle plumes generating OIB [8,12]. 208Pb/204Pb=40.33) resulting from the presence in More recent studies on noble gases have shown the mantle source of subcontinental lithosphere that the two-layer mantle model is confirmed by delaminated during the opening of the Atlantic. For neon isotopic data (e.g. [13–16]). 20Ne and 22Ne are the Terceira end-member, the radiogenic lead isotopic almost primitive isotopes (the nucleogenic produc- signatures (206Pb/204Pb=20.02; 207Pb/204Pb=15.64) tions of 20Ne and 22Ne can be neglected in mantle coupled with the unradiogenic 4He/3He signatures, rocks) and similar 20Ne/22Ne ratios were found for were assigned to mixing between ancient recycled MORB and OIB samples [13–15,17–21]. Different oceanic crust and high 3He material from the lower 21Ne/22Ne ratios are, however, assigned to MORB mantle. The influence of such low 4He/3He plume and OIB sources. Since 21Ne derives from a particle- signature on the Mid-Atlantic ridge basalts was induced nuclear reactions such as 18O(a,n)21Ne demonstrated by [33], who reported a significant [22], its production is connected with radiogenic He decrease of the MORB 4He/3He ratio in the vicinity formation. The higher 21Ne/22Ne ratio observed in of the Azores archipelago. However, given that the MORB as compared with OIB samples (for a given helium anomaly (4He/3He=64,000; R/Ra=11.3) 20Ne/22Ne ratio), is compatible with a higher time- measured in [1] was not sufficient to clearly constrain integrated (Th+U)/He ratio as previously predicted the origin of the Azores hotspot, we decided to analyze from the 4He/3He signatures for the more degassed both helium and neon isotopes in new olivine upper mantle (e.g [14,16]). Moreover, in oceanic phenocrysts sampled in Terceira lavas. Concerning islands where the mixing between primitive mantle Ne, these are the first data published for Azores and MORB sources or magmas is believed to occur, archipelago. it has been shown that Ne can be more sensitive than He in detecting contributions from relatively unde- gassed sources [16,20,23–26]. 2. Geology of Terceira and sample locations Notwithstanding that Azores archipelago emerges in the vicinity of the Mid-Atlantic Ridge, it has been The Azores archipelago is located in the North considered as a hotspot resulting from a mantle plume Atlantic near the triple junction where the Eurasian, [27–29] being characterized by a buoyancy flux African and American plates meet (Fig. 1). The similar to that reported, for example, for the Canary boundary between the Eurasian and African plates is Islands (e.g. [30]). Nevertheless, mantle plumes have currently considered as represented by the NW–SE been the locus of an intense debate and controversy lineament of S. Miguel–Terceira–Graciosa or, alter- still exists concerning their ultimate origin: DQ layer, natively, by the direction defined by S. Miguel–S. bottom of mantle transition zone and even the Jorge Islands. Several models have been proposed to asthenosphere have been considered as possible describe the plate kinematics along this boundary: solutions (e.g. [31]). Noble gas isotope geochemistry pure extension [34–37], pure strike-slip [38] and is a key tool to clear up this question given its ability combined extension and strike-slip (e.g. [39–43]). A P. Madureira et al. / Earth and Planetary Science Letters 233 (2005) 429–440 431 AZORES 42º N 2000m M.A.R 40º N C Fl Terceira ri G Latitude SJ ft F T 38º N P SMi SMa east Azores fracture zone 32º W28º W Longitude Fig. 1. Location of Terceira Island in the Azores archipelago (adapted from [33]). These Islands are the surface expression of a large submarine plateau (M.A.R.—Mid-Atlantic Ridge. Islands from east to west: SMa—Santa Maria; SMi—Sa˜o Miguel; T—Terceira; G—Graciosa; SJ—Sa˜o Jorge; P—Pico; F—Faial; C—Corvo; Fl—Flores). discussion concerning the validity of these models is the volcanism associated with that boundary. As far beyond the aim of our work, but an important stated above, the locus of this extension remains extensional component is obvious taking into account controversial, but some authors argued that it is well Fig. 2. Geological map of Terceira Island (adapted from [44]) with location of the samples analysed in this study. CP—Cinco Picos caldera; GM—Guilherme Moniz Caldera; PA—Pico Alto volcano; SB—Santa Ba´rbara caldera). 432 P. Madureira et al. / Earth and Planetary Science Letters 233 (2005) 429–440 expressed in Terceira Island, in the so-called Terceira basaltic to mugearitic composition (see Table 1 and rift (see Fig. 1; [34–37]). Appendix A). For one sample (PMT-24), a fraction of Terceira Island is slightly elongated in the E–W clinopyroxene was also analysed. Olivine was prefer- direction with an emerged area of about 382 km2. The entially chosen given its significantly lower U and Th relief increases towards west, where the highest contents as compared with clinopyroxene (e.g. [49]) elevation reaches 1021 m, and the topography is preventing the in-situ production of radiogenic 4He dominated by four main strato-volcanoes (Fig.